Arrived dead. All voltages are fine. recapped all the boards. Still no action. There’s no Xtal clock pulse on pin 39 of CPU. Changed Xtal caps…still no action. Checked reset pin 37 and it’s going low on start up then high. Tried a NOS CPU but nothing. It’s got version 1.11 Eprom. Could tge Eprom be corrupted? I checked it on my Gq4x4 and it wasn’t flagged as inoperable, but does that mean anything? Running out of ideas.
As long as there is no clock signal on pin 39, the CPU will not boot.
It is unlikely that the Xtal capacitors have a problem, but I would not say the same about the quartz !
Hi Didier, Thank you. I’ll pop another xtal in and see if I can get the heartbeat back
No go !!! I’m quite confused TBH. The 68B09 chip in the Obi has an internal clocl. I tested the pin which this appears on and it is correctly giving 2Mhz. Interestingly, the chip seems to be ‘working’ in that pins are hi and low and some pluse. Two questions:
- I can not measure a clock on the Xtal dispite swapping the OG for an new Xtal. Should I be able to measure a clock here? What is the role of this Xtal given the species of 68B09 used in the Obi M1000 has an internal clock?
- What’s my next step? The ICs involved in the reset and the address register seem to be ‘functioning’ , whether correctly I don’t know or know how to test. I’ve burned a new EPROM for the OS ( tried the latest OS too) and it made no difference. I seem to have hit the bumpers with trying to fix this and the Korg DVP 1 I have…help please.
The 2 MHz output E (Pin 34) is made from crystal XL1, so if you measure 2 MHz, the crystal is working.
Check the signals on the address and data buses to see if the CPU is running.
If you’ve already replaced EPROM U804, I would check gate U806:C (74HC04), which selects it.
If U806 is OK, I would next check RAM U801.
If all three elements (CPU, EPROM, and RAM) are OK, then check the circuits that drive them, U815:B, U816:A, U814, etc.
Thanks Didier,
That’s great news that the CPU clock is working, which makes sense from my measurments. The only thing I can’t work out is why I can measure the clock coming form an xtal on other synths I have why do you think I can not see the signal on this synth?
When you say ‘check’ signals on address and data busses, what do you mean exactly? I have jad a logic probe on these and they are either in hi or low state or pulsing. Is this how you would check them?
I socketed and replaced 74HC04…nothing happened.
I’ve not done anything with RAM U801. Could you advise me as to what I should try with it?
Thank you. It’s like going back to school for me. I’m OK with analog, but this early digital is a new thing. I love it, but it’s hard going sometimes. I can’t remember how many how to fix arcade game youtube videos I’ve watched 
For a CPU to boot, it needs a processor, an EPROM containing the program, and working RAM, plus a few interfaces to control peripherals.
If the processor is working, there are slots on the address and data buses. If you’ve replaced EPROM U804 and the U806 circuit that selects it, the processor should be able to read the boot program.
After that, it needs RAM to work, hence my comment about RAM U801. Check the circuits involved in generating the /CE signal from the RAM: U814, U815, and U816. And if necessary, replace the RAM.
Good luck and I hope to see you soon !